3GPP Long Term Evolution (LTE) is a standard for mobile phone network technology. LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS), and is a technology for realizing high-speed packet-based communication that can reach high data rates on both downlink and uplink channels.
LTE may be deployed in a number of configurations such as Multiple-Input, Multiple-Output (MIMO) radio systems, including distributed antenna systems. An exemplary MIMO system including a base station and user equipment (UE) is shown in FIG. 1. In MIMO transmission systems, a transmitter sends multiple streams of data by multiple transmit antennas according to frequency and time-domain modulation schemes. Accordingly, in order for the receiver to determine channel impulse responses from each antenna, known reference signals are sequentially transmitted, for instance, as shown in FIG. 2.
Distributed antenna systems ensure cell coverage by deploying many antennas at different locations. In a distributed antenna system, each antenna may transmit on all antenna ports of the cell. However, distributed antennas may be limited to communications on a single antenna port. In this case, different antenna ports may sometimes correspond to physical antennas that are geographically separated from one another.
LTE is regarded as a next generation mobile communication system, but is still relatively young. As such, unforeseen challenges continue to arise in the field as deployment of LTE continues to grow. One particular challenge has arisen with respect to the measurement and reporting of reference signal received power (RSRP) in an LTE network having geographically separated antenna ports.
In an LTE deployment, RSRP provides a cell-specific signal strength metric. This metric is used, for instance, to rank different LTE candidate cells according to their signal strength or as an input for handover and cell reselection decisions. According to the LTE specification, RSRP is defined for a specific cell as the linear average over the power contributions of the resource elements (RE) which carry cell-specific reference signals (CRS) within the considered measurement frequency bandwidth. Further, according to the LTE specification, the CRS transmitted on the first antenna port (i.e. CRS port 0) is normally used for RSRP determination. However, the CRS transmitted on the second antenna port (i.e. CRS port 1) can also be used if available.
In a typical LTE communication system, there are a total of 4 CRS ports for the support of downlink MIMO transmission. Each of the four CRS ports are orthogonal to each other and an example showing CRS port 0 and CRS port 1, having a normal cyclic prefix, is shown in FIG. 2.
Referring now to FIG. 3A, an indoor scenario with interleaved antenna ports, i.e., geographically separated antenna ports, is illustrated. As shown in FIG. 3A, alternating antennas transmit on antenna port 0 (CRS port 0), while the other antennas transmit on antenna port 1 (CRS port 1). An advantage of an interleaved antenna port deployment is an increase in cell coverage, while minimizing the need for additional cabling. This is particularly advantageous when upgrading existing passive distributed antenna systems to support MIMO operation. Use of an interleaved antenna port arrangement, as shown in FIG. 3A, halves the number of antennas used in a distributed MIMO deployment compared to a deployment using two co-located antennas (ports 0 and 1) per site.
However, as shown in FIGS. 3B and 3C and reported in further detail in “Consideration of Real-Life DL MIMO Aspects,” 3GPP TSP-RAN WG1 #64, R1-111330 (2011), depending on the positioning of a user equipment (UE) communication device relative to the locations of interleaved antenna ports 0 and 1, the difference in received reference signal power can be over 25 dB. This is due to, for instance, the disparate path lengths a reference signal travels from each antenna port to the exemplary UE. In contrast, for co-located CRS ports, the RSRP for CRS port 0 and port 1 is generally similar. In FIG. 3C, the RSRP for a UE communication device is plotted based on: 1) the measured value for port 0; 2) an average of the measured values for ports 0 and 1; and 3) the maximum measured value for port 0 or 1. It is clear that the reported RSRP value based on an averaging method has roughly 3 dB of mismatch with the true RSRP value in situations where the UE communication device is close to either antenna port 0 or port 1.
The mismatch between reported and actual RSRP in an interleaved antenna port configuration can have significant side effects with respect to system performance. For instance, cell-coverage may be reduced, hand-off efficiency may be reduced, and data transmission and power settings may not be optimized due to inaccurate, and thus, overly conservative reporting of RSRP. Similarly, overly conservative modulation and coding schemes may be implemented, reducing throughput. Moreover, path loss, which is used in determining uplink power control, is based on the reported RSRP from a UE. Accordingly, interference may be increased due to unnecessarily large uplink transmit power settings that are the result of artificially low reported power values.
Accordingly, there is a need for a method and device for improving performance in a network with geographically separated antenna ports.